A cryogenic readout integrated circuit with analog pile-up and in-Pixel ADC for high frame rate Skipper CCD-in-CMOS Sensors

Abstract

The Skipper CCD-in-CMOS Parallel Read-Out Circuit V2 (SPROCKET2) is an in-Pixel analog front end and ADC designed for the vertical readout of Skipper CCD-in-CMOS image sensors. SPROCKET2 is fabricated in a 65 nm CMOS process and each pixel occupies a \SI{60}{\micro\meter} $\times$ \SI{60}{\micro\meter} footprint. SPROCKET2 is intended to be heterogeneously integrated with a Skipper-in-CMOS sensor ASIC, such that one readout pixel is connected to a multiplexed array of sixteen \SI{15}{\micro\meter} $\times$ \SI{15}{\micro\meter} Skipper-in-CMOS pixels. In order to fully leverage the Skipper CCD-in-CMOS sensor's ability to achieve exceptionally low noise from repeated sampling while minimizing the power consumption of digitization and data movement, SPROCKET2 is designed to ``pile-up'' ten successive samples in the analog front end before digitizing the result with a compact 10-bit SAR ADC at a rate of 66.7 ksps, leading to an effective throughput of 667 ksps. The SPROCKET2 pixel achieves input-referred noise $<100\mu V_{rms}$ with $<1$ LSB ADC non-linearity while consuming an estimated $44\mu W$ per pixel. A SPROCKET2 test pixel was submitted in December 2022, and test results are presented.